“For electrically isolated power supplies, you must determine which side of the primary or secondary side of the electrical isolation controller IC will conduct. If it is located on the secondary side, you must provide electrical isolation to control the primary-side power switch.
Generally speaking, whether it is a controller on the primary side or a controller on the secondary side, in both architectures, a path that can cross the electrical isolation for signal transmission is required, usually an optocoupler (or optical isolator). However, they bring some disadvantages: their rated temperature is usually only 85°C, and the current transfer ratio (CTR) changes over time, which means that their transfer behavior will change during the life of the circuit; in addition, Other components are needed to control the optocoupler. If the optocoupler is used, the feedback loop speed of the isolated power supply is usually very slow.
In recent years, some simple solutions have been developed to solve this problem, such as flyback controllers, which are usually used in applications that require electrical isolation of the power supply voltage and output power below 60W. It does not directly measure the output voltage. By monitoring the voltage across the primary-side transformer winding, a sufficiently accurate criterion about the actual output voltage can be obtained. Its adjustment accuracy depends on the common conditions of the application, including input and output voltages, load changes, and voltage changes.
For many applications, an adjustment accuracy of ±10% to ±15% is sufficient. Figure 1 shows the LT8301. Due to the integrated power switch and SOT23 package, the IC requires very few external components. The isolation breakdown voltage of the circuit depends only on the transformer used, which provides great flexibility, especially When a very high isolation voltage is required.
Figure 1. LT8301 flyback regulator without the need to isolate the feedback path.
However, for applications that require higher output voltage control accuracy, there is another interesting solution-ADI’s flyback controller ADP1071 for the market, which includes a fully integrated feedback path using iCoupler® technology. 2 shows a circuit that requires only a very small number of passive components.
The ADP1071 includes a primary-side controller, a secondary-side active rectifier controller that can improve conversion efficiency, and a fully integrated feedback path to achieve a very fast feedback loop. By adopting this solution, the output voltage regulation is very accurate, and more importantly, very fast, even when the load transient is large, and the allowable operating temperature is as high as 125°C silicon chip temperature.
Figure 2. The ADP1071 flyback controller has an integrated feedback path for very precise regulation.
The maximum isolation voltage depends on the selected transformer and the isolation technology adopted by the switching regulator IC. The maximum isolation voltage of the chip is 5kV, and it has been applied for the reinforced insulation classification level of VDEV0884-10.
Both of the above interesting solutions can be used to develop electrically isolated power supplies. Depending on the application, a solution without a feedback path or a solution with a fully integrated feedback path may be suitable. Because they are no longer restricted by the 85°C operating temperature of the optocoupler, the LT8301 and ADP1071 can achieve very high power density. High compact power supply design.